The kerfing process of a mud jet in assisting the mechanical action of a drill bit is well understood. The drilling mud also lubricates and cools the bit, and is circulated so as to carry away cuttings and rock debris. Normally, drilling mud is directed through a series of conical or tapering nozzles contained in slots above the bit roller cones or defined in the sides of the bit, in a continuous stream.
It is also known that pulsed jets have significant kerfing advantages over continuous stream jets. By exerting alternating loads onto the rock formation pulsed jets may not only produce a high momentary "waterhammer" effect, but may also produce high tensile stress on the compression strength of the formation. This would give rise to the weakening of the formation through the reflection of stress waves, prior to any mechanical shearing, gouging, or scraping action of the drill bit, leading to faster removal of debris and faster penetration rates.
However, a downhole tool that produces a pulsed jet through mechanical interruption or mechanical excitation of the normal or steady flow of drilling fluid would cause large energy losses, as well as mechanical wear on the indispensable moving parts and seals. Oscillating valve arrangements to cause flow pulsing are described, for example, in European Patent Specification Nos. 0,333,484A and 0,370,709A. A nozzle is described in British Patent Specification No. 2,104,942A for restricting flow and inducing cavitation, i.e. the formation of bubbles in the fluid which implode on contact with the rock formation, which weakens and erodes the surface being drilled. However, in order to improve removal of rock debris, fluid is also directed at higher pressure through a non-cavitating nozzle to provide a cross flow. It will be appreciated that a single nozzle delivering a rapidly oscillating pulsed flow would achieve these effects more efficiently.
A self-excited, acoustically resonating nozzle causing the emitted jet to be structured with large discrete vortex rings is described by V. E. Johnson, Jr. et al (ASME Journal of Energy Resources Technology, Vol. 106, June 1984, p. 282-288). A nozzle with a reduced diameter "organ pipe" section for creating acoustically resonant standing waves inside the nozzle induces excitation and structuring of the jet outside the nozzle, which can also be accompanied by cavitation. However, this proposal does not suggest that self-excited oscillation of the jet may be induced inside the nozzle, so as to produce a rapidly pulsating jet as it emits from the nozzle. Furthermore, a problem associated with acoustically resonating nozzles is that the length of the nozzle is limited by the space available in the bit plenum for locating the nozzles. Nozzle extensions are also subject to breakage and failure down hole.
A nozzle for the self-excited oscillation of a Newtonian fluid such as water, producing a pulsed jet for brittle material cutting applications has been investigated by Z. F. Liao and D. S. Huang (Paper 19, 8th International Symposium on Jet Cutting Technology (1986) Durham, England). The nozzle comprises a simple axisymmetric cavity with an inlet and an outlet orifice of smaller diameter than the cavity diameter. Periodic pressure pulses are generated in the shear layer between the jet in the cavity and the surrounding fluid, and the jet oscillates as it emits from the nozzle to atmosphere. However, there is no teaching of a similar effect in a non-Newtonian or thixotropic fluid such as drilling mud, emitting from a nozzle to a high pressure fluid environment as opposed to ambient air.